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Ill. 5: Bernstein database architecture
Architecture
After detailed discussions with the different partners in the
project, it was decided that a new large database with
copies of the existing databases should not be created, but
searches should take place directly in the original databases.
For this approach various protocols were investigated,
and with “Search/Retrieval via URL” (SRU) an appropriate
solution was found. To realise this protocol, a SRU-gateway
was implemented that could be easily configured for each
database. It was specified that MySQL- und Microsoft Access-databases
should be supported by this SRU-gateway
(Ill. 5).
As shown in the figure above, the SRU-gateway can be
installed either near the original database (dashed line) or
directly on the Bernstein server (dashed dotted line).
SRU/SRW Protocol
SRU (Search/Retrieval via URL) und SRW (Search/Retrieve
Web Service) are standard search protocols for Internet
search queries. They were developed and published by the
United States Library of Congress (see http://www.loc.gov/standards/sru/).
The actual requests use CQL (Contextual
Query Language), a standard syntax for representing
queries and the results are returned in XML (eXtensible
Markup Language).
Since August 2007, SRU version 1.2 has become the current
standard version; it no longer distinguishes between
the SRU and SRW protocols, but includes both.
The search queries are sent via HTTP (HyperText Transfer
Protocol) using simple GET or POST requests, or enveloped
in XML using SOAP (originally for Simple Object Access Protocol).
The following operations are specified in the SRU protocol:
• explain: provides a description of the facilities available
on the SRU server
• scan: enables the range of available terms at any given
point to be listed in ascending order
• searchRetrieve: allows a search to be submitted and a request
to retrieve matching records in a specific sort order
CQL allows a search with logical operators (AND, OR, NOT)
and numerical relations (=,?, , =, =), as well as an exact
search and a search for substrings. Furthermore, the sort order
of the results can be specified in advance.
The response to a search request is returned in XML. As
default schema, “Dublin Core” with fifteen elements is
used. A response can either consist only of the number of
hits, or can include an explicit number of matching records
starting with a specific one.
SRU Gateway
The SRU-gateway was developed as a servlet in Java and is
therefore independent from the platform. All the functions
of the SRU version 1.2 that are necessary for searches in
watermarks databases have been implemented. The technical
requirements are a Java runtime environment (JRE =
1.6_3), a servlet container (Apache Tomcat = 5.5.23) and
read access to the respective MySQL- or Microsoft Accessdatabase.
The individual databases are configured by the use of an
XML file (“config.xml”) that includes the data for the database
access and the mapping of all database fields. The following
example shows an extract of the configuration file
for the WZMA database:
bernstein_wzma_g.motif_long
bernstein_wzma_m.refnr_wm
bernstein_wzma_m.parA
bernstein_wzma_m.parH
bernstein_wzma_m.parW
bernstein_wzma_m.origin
bernstein_wzma_m.date_begin
bernstein_wzma_m.date_end
bernstein_wzma_m.source
bernstein_wzma_m.path_wm
bernstein_wzma_m.id
http://www.ksbm.oeaw.ac.at/imgjpg/%img
path%%dcx.refnr%.jpg
http://www.ksbm.oeaw.ac.at/_scripts/php/loadWmarkImg.php?id=%id%
In the case of a “searchRetrieve” request, the transmitted
fieldnames are first mapped onto the fieldnames of the
specific database according to the configuration file, and
the CQL is transformed into SQL. Then the individual items
of the result set are transformed into XML with regard to
the mapping (and according to the SRU protocol) and returned.
In the following example we assume that an advanced
search for the motif “vogel krone” was executed via the
Bernstein portal, which results in 107 hits in the Piccard-Online
database:
103